[SOLVED] Intro course Twin Paradox 1. The problem statement, all variables and given/known data You probably already know. One twin stays on earth. One goes 4ly away at .8c. The question asked a bunch of different things. How long does the twin on the ship think it takes. How long does the twin on earth think it takes. How long does the twin on the ship measure the trip to be (length not time). 2. Relevant equations SEE BELOW 3. The attempt at a solution My issue is conceptual. The proper time is the frame where both events occur at the same place correct? So shouldn't the twin on the ship measure proper time of 4ly/.8c = approx. 5 years. So then time dialation says on earth they'd measure 5years/.6 or about 8.33 years? But then they say what length does the twin on the ship measure. Wouldn't it be 4ly? Because for 5 years he travels at .8c? But earth would also measure 4 ly would it not?